A turbomachine of this type has two external fans mounted for contrarotation about a common axis, constituting respectively an upstream fan and a downstream fan, each fan being constrained to rotate with a turbine of the turbomachine and each extending substantially radially outside the nacelle of the turbomachine. Such a turbomachine has the advantage of providing very high performance compared with other types of turbomachine since it consumes less fuel and since its contrarotating fans enable it to deliver a high level of thrust.
Nevertheless, a major drawback of that type of turbomachine is the noise that it generates in operation. Unfortunately, such a turbomachine must comply with relatively severe acoustic certification standards, in particular during the takeoff and landing stages of an airplane fitted with such a turbomachine.
One of the sources of the noise comes from vortices that are generated at the tips of the blades of the upstream fan interacting with the blades of the downstream fan.
One solution for eliminating the noise is to reduce the outside diameter of the downstream fan so that the vortices generated by the upstream fan pass outside the downstream fan and do not interact therewith. That solution is known as “clipping”. Nevertheless, that solution is not satisfactory in that it leads to a reduction in the thrust produced by the downstream fan and thus to a reduction in the performance of the turbomachine. It would be possible to increase the load on the downstream fan in order to compensate the reduction in its diameter, but then the fan would become very complex to make.